Both the Electronic Industries Association (EIA) and the International Telegraph and Telephone Consultative Committee (CCITT, also called ITU) have established engineering standards to facilitate the interconnection of equipment used for the serial transmission of digital data. These standards specify mechanical characteristics, electrical characteristics, and protocols for digital data exchange. There are many different standards used to interconnect DTEs and DCEs. Some of these standards are used world-wide while others are regional.
The standards discussed herein can be divided into two general classes of communications interfaces: single-ended, unbalanced, common return digital interfaces, and balanced, differential digital interfaces. The EIA RS-232 standard specifies a single-ended, unbalanced interface generally limited to transmission rates of up to 20,000 bits per second over a 50 foot cable. The EIA RS-449 standard and CCITT standard X.21 provide a balanced differential interface that is capable of transmission rates of up to 10,000,000 bits per second over a 25 foot cable. The CCITT V.35 standard is an older type of interface that uses a mixture of balanced, differential signals and single-ended unbalanced signals, and is limited to transmission rates of up to 64,000 bits per second over a 50 foot cable.
With the multiplicity of interface standards in use it is difficult to design DTEs that will interconnect with the wide range of available DCEs. Most existing DTE interconnection devices Support a single type of interface. Typically, different interface standards are accommodated by requiring the user to interchange internal circuit boards or components.
Several attempts have been made to develop a DTE interface that will support multiple standards. However these DTE interfaces require the user to switch internal circuit connections or add adapter circuit modules to accommodate to a different interface standard. An example of this type of interface is described in the Sealevel Systems Model 4013 Communications Board Functional Specification and User Manual (Sealevel Systems, Liberty, N.C.).
One DTE board supporting multiple standards is disclosed in U.S. Pat. No. 4,403,111 (the '111 patent) to Kelly. This patent discloses a DTE interface that permits the DTE to provide the EIA RS-232 interface, the EIA RS-449 interface, and a proprietary interface, which is a derivative of the EIA RS-449 standard, using only a single 25 pin connector. However, the DTE interface disclosed in the '111 patent omitted some critical signals and hence did not fully implement the applicable standards. In addition this device failed to present the required termination impedance to the connected DCE. Further the DTE interface disclosed in the '111 patent was not capable of accommodating more than 2 different communications standards due to pin limitations in the interface connector.
The present invention relates to a DTE interface is desired that will interconnect with a range of different DCE's using different interface standards, without requiring that the user take special actions to permit the interface to properly function.